A potential role for epigenetic mechanisms enabling appropriate seasonal reproductive transitions of liver yolk‐precursor production

Animals breed at times of the year that ensure offspring production and growth during favorable periods. DNA methylation is one mechanism by which expression of genes necessary for reproduction may be regulated, enabling expression only at appropriate times. Much work on seasonal breeding in vertebrates has focused on the neuroendocrine system, however oviparous vertebrates, including birds, also rely on the liver for production of yolk precursors, such as vitellogenin (VTG) that will provide the nutrients necessary for development in ovo. We hypothesized that changes in DNA methylation in the promoter for VTG2 in the liver may be one mechanism ensuring appropriately timed seasonal breeding. DNA methyltransferases (DNMTs) facilitate de novo (DNMT3a) and maintenance (DNMT1) of DNA methylation. We observed that liver expression of VTG2 was lower in birds sampled during the pre‐breeding compared with the early‐breeding period, and also observed changes in liver expression of DNMT1 and DNMT3a between these two periods. Contrary to our predictions, we observed an increase in methylation from pre‐ to early‐breeding at one of three CpG sites in the promoter of the VTG2 gene, with no differences at the other two CpG sites. Finally, we asked if patterns of DNA methylation of VTG2 in liver were similar in the blood. Although we observed strong correlations between blood and liver in two sites that did not change between pre‐ and early‐breeding, there was only a trend for a significant association between blood and liver DNA methylation at the site that displayed an increase in liver DNA methylation between sampling periods. Together, these findings suggest that changes in DNA methylation in an important tissue outside of the reproductive endocrine axis (liver) may play a critical role in appropriate timing of seasonal clutch initiation, though it is unclear if these epigenetic changes are all reflected in blood.